i like how most people are immediately starting the discussion with ms abilities, when have we ever seen either of these 2 shinobi go straight to ms in a fight besides kakashi tanking susanoo arrows? :sy:
that said kakashi's canon kamui usage to date does level the playing field because it may no longer be a last resort for him, but then again neither was tsukuyomi/amaterasu for itachi, but most assume the fight will make it too that point, the way i see it , it would mostly be a 3 tomoe battle with the final couple of strategic moves revolving around each persons ms, given that itachi has an upper hand, he weaves seals faster than kakashi's sharingan can read them, thats a severe advantage in a battle with 2 fairly evenly matched shinobi, taijutsu theyre pretty even from what i can tell, and itachi is still a genjutsu master so you have to take that into account.
kakashi however has a wide arsenal and 3 nature manipulations to counter the ninjutsu's itachi would throw at him, so i believe he has some advantage in jutsu usage, but in the end it would come down to ms usage and theres no way for itachi to counter kamui besides clones, which kakashi would be aware of, and could play to his advantage
i think itachi has a higher probability of winning but its not too large of a margin, six out of ten times id think itachi would win a battle between these 2, which is an improvement from what i thought before recent developments in the manga